1. Field of the Invention
The present invention relates to a sheet processing apparatus and, more particularly, to a sheet processing apparatus having a sheet buffer function.
2. Description of the Related Art
In recent years, in some image forming apparatus, a sheet processing apparatus is set adjacent to a paper discharge unit of an image forming apparatus main body in order to perform finishing for sheets after image formation such as sorting of the sheets or stapling the sheets.
The sheet processing apparatus has plural means for conveying a sheet, which is conveyed from the image forming apparatus, to a paper discharge tray and discharging the sheet. The means are roughly divided into a conveying path for not performing the finishing and a conveying path for performing the finishing. When the finishing is not performed, the sheet is conveyed through the conveying path for not performing the finishing and directly discharged onto the paper discharge tray. When the finishing is performed, the sheet is conveyed to a processing tray through the conveying path for performing the finishing, which is branched from the conveying path for not performing the finishing, and stacked. When a set number of sheets are stacked, the sheets are aligned on the processing tray and subjected to the finishing.
Concerning sorting, for example, there is proposed a sheet processing apparatus including a paper discharge roller that discharges a sheet, a roller that nips the sheet between the roller and the paper discharge roller, and shifting means for moving, in a state in which the sheet is nipped between the paper discharge roller and the roller, the sheet in a direction orthogonal to a sheet conveying direction by a predetermined distance and discharging the sheet at a point when the sheet is moved by a predetermined amount, wherein the sheet processing apparatus alternately discharges each of sheets in an initial position before the shifting means is actuated and a shifted position after the shifting means is actuated and performs sorting of the sheets (e.g., JP-A-2002-114428). On the other hand, noise caused in the sorting of the sheets is regarded as a problem.